In recent years, as conventional oil and gas reserves are declined gradually, the exploitation and utilization of non-conventional energy resources has received more and more attention.
Among non-conventional oil and gas energy resources, shale gas, which is low-carbon and environment friendly, intensive and efficient, has receive wide attention internationally. The total reserve of shale gas in the world is 207××1012 m3, and is mainly distributed in USA, China, Middle East and North Africa regions, wherein the total reserve of shale gas in China is 31.6×1012 m3, and is in the first place in the world. In USA, since the energy independence initiative and shale gas revolution are carried out successfully, shale gas has been exploited industrially on a large scale, and USA has got rid of the situation of oil and gas import. The economical and green shale gas resources not only have made substantive contributions to the economic rehabilitation and growth in USA, but also have brought inspirations to the energy deployment in the countries around the world.
Shale rocks are sedimentary rocks form under multiple actions in a long time, and the composition of shale rocks is complex. In the early diagenetic stage of shale rocks, both hydration and dehydration happen; in the late diagenetic stage, dehydration mainly happens. A large quantity of clay minerals exist in shale rocks, and the crystal structure of the clay minerals mainly consists of two basic structural units, i.e., silicon-oxygen tetrahedron and aluminum-oxygen octahedron. The types of the clay minerals are determined by different stacked structures of tetrahedral layers and octahedral layers. The high-valence cations Si4+ and Al3+ in the crystal lattices of clay minerals are often substituted respectively by low-charge ions such as Al3+, Fe2+ and Mg2+, etc., and consequently the mineral particles exhibit certain electronegativity. However, clay minerals are neutral owing to the fact that the low-valence cations such as K+, Na+ and Ca2+, etc. in the strata temporarily make up for the positive charges of mineral particles under an action of attraction between positive charges and negative charges in the clay mineral formation process. Nevertheless, clay minerals exhibit their electronegativity in water because those ions may be dissociated and liberated from the clay minerals in water. The most typical clay mineral in shale gas reservoirs is montmorillonite (bentonite), which is a typical hydrated swelling mineral, mainly consists of silicon-oxygen tetrahedrons and aluminum-oxygen octahedrons, and is formed by repeatedly stacked structural units, each of which is composed of two silicon-oxygen tetrahedron layers and one intercalated aluminum-oxygen octahedron layer; the crystal layer structure is of 2:1 type, oxygen atoms exist between the structural unit layers, the bonding force between adjacent crystal layers is mainly Van der Waals force, and no hydrogen bond exists between the layers; as a result, the bonding is very weak and may be broken easily, water can enter into the spaces between the layers easily, and cation substitution and exchange may happen easily. After the cations between the crystal layers are substituted, the surfaces of the crystal layers exhibit electronegativity and therefore are mutually repulsive, encouraging external water to further enter into the space between the layers.
At present, usually oil-based drilling fluids are used in shale gas well drilling to avoid water intrusion into the shale gas formation. Though those drilling fluid systems have advantages including well wall stabilization, good lubrication property, and high temperature resistance, etc., oil-based or synthetic based drilling fluids usually involve high cost, severe contamination, and complex post-treatment in actual applications. In the meanwhile, domestic and foreign researchers have carried our extensive and successful researches and applications on high-performance water-based drilling fluid systems (e.g., amines). Most amine polymer inhibitors that have been developed up to now are in linear structures. Most linear polymers are in a state of random coils or linear structures after they are dissolved in drilling fluids, and they are absorbed to and wind on the surfaces of shale rocks in a random and non-uniform manner; there are less adsorption sites relatively, and repetitive multi-layer adsorption or zero polymer adsorption may occur locally. Besides, under the turbulent scouring action of drilling fluid, a linear inhibitor may be desorbed easily, and consequently the surfaces of shale rocks where no polymer is absorbed may still be hydrated and swell easily, finally resulting into wellbore instability or deteriorated drilling fluid properties.
Hyperbranched polymers are totally different from linear polymers in terms of structure and performance, owing to their highly branched mesh structures and a large quantity of terminal functional groups. Hyperbranched polymers are absorbed to and wind on the surfaces of shale rocks uniformly, and there are more adsorption sites relatively.
Therefore, there are certain application prospects for utilization of amino-terminated hyperbranched polymer inhibitors to improve shale hydration inhibiting capability and thereby improve overall inhibition performance of drilling fluids.